The present invention relates to a secondary battery using a positive electrode active material, a negative electrode active material each formed of a material capable of charge-discharge reaction by insertion-release of protons, and a proton conductive electrolyte, said secondary battery being excellent in safety and reliability, capable of taking out a large current, and excellent in cycle life. To be more specific, the present invention relates to a secondary battery characterized in that a specific polymer containing a quinoxaline structure is used in the positive electrode active material and/or a negative electrode active material.
Sale of new secondary batteries such as a nickel-hydride battery and a Li ion secondary battery, which have a high energy density and, thus, rapidly have come to be mounted to a small portable equipment, is rapidly increased in recent years. Particularly, use of a Li ion battery further promotes miniaturization in weight, size and thickness of the equipment and, thus, the Li ion battery now constitutes the main article of the secondary batteries. For example, vigorous researches are being conducted on a lithium ion battery comprising a positive electrode containing a metal oxide or a metal sulfide such as LiCoO2, LiNiO2, LiMn2O4 or MoS2, a negative electrode containing lithium, a lithium alloy or a carbon material or an inorganic compound capable of absorbing-desorbing lithium ions, and an organic electrolyte. A lithium battery comprising a positive electrode containing LiMn2O4, LiNiO2 is reported in xe2x80x9cJ. Electrochem. Soc., Vol. 138, No. 3, page 665, 1991xe2x80x9d.
There are many reports on the battery using a conducting polymer as an electrode active material. For example, a lithium secondary battery using polyanilines in the positive electrode has been put on the market by Bridgestone/Seiko Inc. as a coin type battery for use in a back-up battery, as reported in xe2x80x9c27-th Battery Symposium, 3A05L and 3A06L, 1986xe2x80x9d. Also, it is proposed to use polyaniline, which is capable of oxidation-reduction by proton, as a positive electrode active material of a battery using an acidic aqueous solution (Bull. Chem. Soc. Jpn. 57, page 2254, 1984).
However, since a lithium-based battery uses lithium and/or a lithium compound that is active in water and air and, thus, is oxidized easily, problems such as safety and reliability in the cases of short-circuiting, high temperature, liquid leakage or unsealing are worried about. Therefore, counter measures for safety are taken by various methods such as an improvement of the separator, incorporation of a PTC element and sealing. Recently, various studies for using the polymer solid electrolyte exhibiting a lithium ion conductivity in place of an organic electrolyte solution are being made in an attempt to improve the safety and reliability. A battery of this type has now been partly put on the market. A battery using a solid electrolyte containing a polymer as a main component is more flexible than a battery using an inorganic material and, thus, produces a merit that the battery can be worked into various desired shapes. However, the battery studied up to now is defective in that the polymer solid electrolyte is low in its lithium ion conductivity, leading to the problem that the taken-up current is small.
The present inventors previously proposed in JP-A-10-289617 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent application (Kokai)xe2x80x9d) a proton migration type secondary battery excellent in safety, reliability and current characteristics and having a long life and a large capacity in an attempt to improve the defects of the new type batteries such as the lithium ion batteries described above and, thus, to improve the safety, the rapid current characteristics and the like. Proposed as the electrode active materials of these batteries are polypyridine series and/or polypyrimidine series and/or sulfonic acid side chain series and/or hydroquinone series polymer and/or manganese oxide. Since the proton insertion-release can be performed easily in these materials, it was possible to obtain a secondary battery excellent in safety and its rapid current characteristics. However, since the capacity of the proton insertion-release is insufficient, the battery was markedly inferior to the conventional new type battery in the energy density of the battery.
In recent years, widely used in a memory back-up power source or the like is an electric double layer capacitor disposed of an ionic conductive solution (electrolyte solution) that is interposed between polarizable electrode materials consisting of a carbon material having a large specific surface area such as activated carbon and carbon black. For example, an electric double layer capacitor using an aqueous solution of sulfuric acid is proposed in xe2x80x9c173rd Electrochemical Society Meeting, Atlanta, Ga., May 1988, No. 18xe2x80x9d. The electric double layer capacitor available on the market includes a capacitor using an organic electrolyte solution and a capacitor using an acidic aqueous solution such as sulfuric acid. The aqueous solution type is certainly low in its energy density. However, the electrolyte solution used in the capacitor of this type exhibits a high ionic conductivity and, thus, the capacitor can be charged and discharged at a high speed, leading to excellent rapid current characteristics.
It is reported in xe2x80x9cJ. Electrochem. Soc., Vol. 145, No. 4, page 1193, 1998xe2x80x9d that polyphenyl quinoxaline exhibits an oxidation-reduction reaction within an acidic aqueous solution. However, this literature does not suggest the idea of utilizing the polyphenyl quinoxaline in a proton migration type battery. Of course, this literature does not suggest the idea of the present invention that polyphenyl quinoxaline is used in a negative electrode.
An object of the present invention is to provide a large capacity proton migration type secondary battery excellent in safety, reliability and rapid current characteristics, and having a long life. Another object of the present invention is to provide an electrode material and/or an electrolyte material that exhibits excellent characteristics when used in the secondary battery.
As a result of an extensive research conducted in view of the situation described above, the present inventors have found that a polymer containing a quinoxaline structure, which is used as an electrode active material, exhibits a large insertion/release capacity of protons, and that a proton migration type secondary battery using the particular polymer is excellent in safety, reliability and rapid current characteristics. It has also been found that the particular proton migration type secondary battery exhibits a long life and a high weight energy density (kWh/kg), compared with the conventional aqueous solution type double layer capacitor and the lead acid battery using sulfuric acid.
The present inventors have found that a proton migration type secondary battery excellent in productivity, safety, and reliability can be obtained by using such an electrolyte as a solid and/or gel electrolyte obtained by curing a mixture consisting of a polymerizable compound excellent in its polymerizing properties and an electrolyte exhibiting a proton conductivity.
The present inventors have also found that a proton migration type secondary battery having a further improved life and excellent in reliability can be obtained by adding a non-electrically conductive powder material to the electrolyte.
That is, the objects given above have been achieved in the present invention by developing a secondary battery given below:
1) A material comprising a polymer having a quinoxaline structure and being capable of charge-discharge reaction by insertion-release of protons.
2) The material comprising a polymer having a quinoxaline structure and being capable of charge-discharge reaction by insertion-release of protons as described in the above 1), wherein the polymer having a quinoxaline structure has a quinoxaline skelton as a repeating unit represented by the following formula (1): 
wherein each of R1 to R4 independently represents a hydrogen atom; a hydroxyl group; an alkyl group, which may have a hetero atom, having 1 to 20 carbon atoms; an alkenyl group, which may have a hetero atom, having 2 to 20 carbon atoms; an alkynyl group, which may have a hetero atom, having 2 to 20 carbon atoms; an aryl group that may have a substituent group; a hetero aryl group that may have a substituent group; a carboxyl group; or a carboxyalkyl group having 2 to 10 carbon atoms, which may be linear, branched or cyclic.
3) The material comprising a polymer having a quinoxaline structure and being capable of charge-discharge reaction by insertion-release of protons as described in the above 1), wherein the polymer having a quinoxaline structure has a quinoxaline skelton as a repeating unit represented by the following formula (2): 
wherein each of R5 to R8 independently represents a hydrogen atom; a hydroxyl group; an alkyl group, which may have a hetero atom, having 1 to 20 carbon atoms; an alkenyl group, which may have a hetero atom, having 2 to 20 carbon atoms; an alkynyl group, which may have a hetero atom, having 2 to 20 carbon atoms; an aryl group that may have a substituent group; a hetero aryl group that may have a substituent group; a carboxyl group; or a carboxyalkyl group having 2 to 10 carbon atoms, which may be linear, branched or cyclic.
4) The material comprising a polymer having a quinoxaline structure and being capable of charge-discharge reaction by insertion-release of protons as described in the above 1), wherein the polymer having a quinoxaline structure has a quinoxaline skelton as a repeating unit represented by the following formula (3): 
wherein each of R9 to R16 independently represents a hydrogen atom; a hydroxyl group; an alkyl group, which may have a hetero atom, having 1 to 20 carbon atoms; an alkenyl group, which may have a hetero atom, having 2 to 20 carbon atoms; an alkynyl group, which may have a hetero atom, having 2 to 20 carbon atoms; an aryl group that may have a substituent group; a hetero aryl group that may have a substituent group; a carboxyl group; or a carboxyalkyl group having 2 to 10 carbon atoms, which may be linear, branched or cyclic; Ar is a divalent aryl group that may have a substituent group or a divalent hetero aryl group that may have a substituent group; p is an integer of 1 to 5; X is a hetero atom, a divalent aryl group that may have a substituent group or a divalent hetero aryl group that may have a substituent group; and k is an integer of 0 to 5.
5) An electrode for a battery capable of charge-discharge reaction by insertion-release of protons, characterized by comprising the polymer recited in any one of items 1) to 4) and an electrically conductive carbon material.
6) The electrode for a battery according to item 5), wherein the electrically conductive carbon material is a fibrous carbon material.
7) A secondary battery, in which a positive electrode active material and/or a negative electrode active material is a material capable of charge-discharge reaction by insertion-release of protons, and an electrolyte exhibits a proton conductivity, characterized in that the material recited in any one of items 1) to 4) is used as the negative electrode active material.
8) The secondary battery according to item 7), wherein the electrolyte is a proton conductive solid and/or gel electrolyte.
9) The secondary battery according to item 8), wherein the solid and/or gel electrolyte is obtained by hardening a mixture consisting of a polymerizable compound having a double bond and a proton conductive substance.
10) The secondary battery according to any one of items 7) to 9), wherein powder of at least one kind of a non-electronically conductive material is contained in the electrolyte.
11) The secondary battery according to item 10), wherein the powder of at least one kind of the non-electrically conductive material consists of inorganic fine particles having a primary particle diameter of about 0.001 to about 10 xcexcm.